With the rapid development of modern technology, new generations of broadcasting technologies have changed dramatically. For example, unlike the analog television, digital television broadcasting not only requires the stability of fixed reception, but also requires the same quality in mobile reception, such as via smartphones, or other hand-hold mobile receivers for real-time TV signals. This has raised a much higher standard towards the signal coverage and also the transmitted signal quality. However it's not easy for wireless or broadcasting network to establish a good coverage in every spot within a large area, because the coverage is limited by many factors, such as transmitted power from TV tower, size of coverage area, terrain obstacles for signals like being blocked by hills or mountains, or being reflected by lakes or seas, not to mention many man-made factors like skyscrapers and moving vehicles in cities. The transmitted signals can be greatly degraded by the multipath interference in all the above situations. Even with the increase of power level in a single transmitter, there may be still some areas which have insufficient signal strength or even are totally blind. Therefore, to improve signal coverage in those areas has become the main concern of network construction in the future.
By far the most common way is to use On Channel Repeater (OCR), which may also be called Gap filler. It's low cost and has a much easier and quicker installation when compared to adding an additional high power transmitter, or establishing a single frequency network (SFN), so it's a good way for coverage extension, especially for small areas. On Channel Repeater is a rebroadcast transceiver, which receives RF signals with a receiving antenna and then complete signal processing, such as to amplify the received signals, etc., and finally to use a transmitting antenna to retransmit the signals at the same frequency as it's received, towards those area with poor signal coverage. OCR has many advantages such as it has a simple structure, small delay and low cost, which makes it the perfect solution for directional coverage such as to roads, tunnels and subways, or communities and villages.
Due to the transmitting frequency and receiving frequency in an OCR are the same, it may probably cause a self-exciting oscillation. This is how it happens: if there is not enough isolation between the transmitting antenna and the receiving antenna, then the retransmitted signals are most likely to be received by the receiving antenna of the OCR again. This “unwanted” signals (we call them echoes in this invention), together with the “wanted” signals, which are supposed to be received and retransmitted, are to be amplified, then retransmitted via the transmitting antenna. These echoes make a closed loop in the OCR therefore it causes the instability and relaxation oscillations. In order to avoid a self-exciting oscillation, usually some physical isolation methods are adopted during the initial design and implementation of projects, such as the use of distance difference, altitude difference, or the height ratio and polarization difference of receiving and transmitting antennas. However these physical ways usually greatly increase the difficulty of the engineering design and construction cost, and what's worse, sometimes it still cannot create sufficient isolation purely by all the physical ways mentioned above.
Echo cancellation technologies are then introduced in order to make up for the insufficiency of physical isolation. Although there are various of echo channel estimation and cancellation methods, they are similar in terms of general structure: the OCR receives mixed signals which consist of “wanted” signals and echoes, then it down converts the mixed signals to baseband or low IF and converts them to digital signals. After that it uses digital signal processing technologies to complete echo estimation and echo cancellation. Finally after echoes are substantially eliminated, it converts the signals back to be analog, up converts them to the same frequency as it's received, amplify them, and retransmit them via a transmitting antenna, as shown in FIG. 1.
A Chinese patent whose application number is 200810106326.7, “An Echo Cancelling Apparatus and its Echo Cancellation Method”, is used in mobile multimedia broadcasting field. Its feature is that it does not use any training sequence for echo channel estimation; therefore it does not bring in extra noise which may degrade the performance of receivers. But its essence in the structure is similar to other echo cancellation technologies: they first down convert the mixed signals to baseband and then use digital signal processing for echo estimation and echo cancellation. In all other inventions which have similar structure, as shown in FIG. 1, the echo cancellation performance is not only affected by the method used for echo detection and cancellation, by also greatly limited by the dynamic range of the ADCs they uses. Let's say they use a 10-bit ADC in OCR, which is commonly used in the field, and its valid dynamic range is only 50 dB. If echo level is 30 dB higher than the wanted signals, then even the apparatus can detect and eliminate most of the echoes precisely, the remaining wanted signals have a valid range of only 20 dB, while all the other parts are just noise. And after frequency up conversion and power amplification, the quality of retransmitted signals is to get worse. These signals of poor quality retransmitted by an OCR will greatly degrade the performance of receivers. However since the deterioration is done in analog domain caused by the limited dynamic range of the ADC, it cannot be recovered by any simple digital signal processing technologies in digital domain which means the moment these signals are converted from analog to digital, the damage is done and irreversible. To replace the regular ADC with a higher resolution one which has higher dynamic range (e.g., from 10-bit to 14-bit) may help to relieve the problem in theory, but all come with it is a much higher cost, more power consumption and less generality, and the improvement is very limited in some cases.